Class II MHC (la) gene products play critical roles in a variety of T and B lymphocyte responses. A combination of immunological, molecular genetic, and biochemical approaches is being used to study the relationship between la structure and function. Sequential intracellular folding and targeted intracellular transport of class II molecules is central to the function of la in the acquisition of antigenic peptides for presentation to T cells. These events have been examined using spleen cells and cells transfected with mutant class II alpha and beta chains. Analysis of surface expression has revealed a requirement for 'accommodation' of the highly polymorphic regions characteristic of MHC proteins for efficient transport of assembled alpha beta dimers to the cell membrane. The interacting regions have been mapped using recombinant alpha and beta chains, and the data provide support for the current structural model of class II molecules based on the class I HLA crystal structure. These studies also have demonstrated that the defect in expression in most cases of "mismatched" alpha and beta chains lies in the failure of assembled heterodimers to efficiently exit the endoplasmic reticulum. Many of these poorly transported dimers can be 'rescued' at least in part by co-expression of adequate amounts of the non-MHC encoded invariant chain. Additional studies have revealed that la heterodimers adopt a series of conformational states during intracellular transport and that the folding state of la molecules is controlled by association with invariant chain and with antigen. The use of direct peptide binding techniques, together with transfectants expressing mutant class II molecules, has permitted the definition of a local peptide binding subregion in the murine I-E molecule, and the role of specific allelically polymorphic residues in controlling the quantitative and qualitative binding of peptide to this molecule. Finally, mutational analysis has begun to define the site(s) of interaction of la with CD4, a molecule that functions as a co-receptor along with the T cell receptor in recognition of class II molecules during T cell activation. Together with the studies on class II folding, transport, and control of peptide binding by the polymorphic domain, these experiments will provide new insight into the molecular mechanisms involved in antigen recognition by and activation of T lymphocytes.